Whether 10 km eggs hatch into lower rarity Pokemon is a controversial topic. Many websites says those eggs can hatch into Pokemon like Pidgey, but players deny that based on their experience.
Some websites argue that 10 km eggs hatch into lower rarity Pokemon, but players deny that argument based on their actual experience of hatching eggs. It was a controversial topic and has confused beginners.
However, according to the post of Poekmon Go forum in Japan, 10km eggs seem to never hatch into Pokemon like Pidgey. They hatched many 10km eggs, but all of those eggs become Pokemon which are considered to hatch from only 10km eggs.
In Japan, one player hatched nine of 10km eggs at once as the screenshots below. From those eggs, he got the following Pokemon:
Pinsir ×1
Dratini ×1
Kabuto ×1
Eevee ×3
Scyther ×1
Chansey ×1
Magmar ×1
10km eggs ×21
A player hatched 21 eggs got Pokemon as the following table. There is no Pokemon which can hatch from 2km eggs or 5km eggs. All of them are considered to be gotten from only 10km eggs.
Another player got the followings from 12 eggs, which require 10km to hatch. As well as the first player, there are only Pokemon which hatch from 10km eggs.
The third player captured the following Pokemon by hatching 7 eggs (10km). As same as two above, there are no Pokemon which hatch from 2km or 5km eggs.
As this page has shown, 10km eggs never hatch into Pokemon hatch from other tier eggs: 2km and 5km. That is obvious from the result of hatching 10km eggs × 10. Our website, GameA, update the following articles, the lists of Pokemon from each eggs, soon based on this study!